Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba, JAPAN.
Laboratory for Exercise and Environmental Physiology, Faculty of Education, Niigata University, Niigata, JAPAN.
Med Sci Sports Exerc. 2021 Apr 1;53(4):845-852. doi: 10.1249/MSS.0000000000002537.
Caffeine is an exercise performance enhancer widely used by individuals engaged in training or competition under heat-stressed conditions. Caffeine ingestion during exercise in the heat is believed to be safe because it does not greatly affect body temperature responses, heart rate, or body fluid status. However, it remains unknown whether caffeine affects hyperthermia-induced hyperventilation or reductions in the cerebral blood flow index. We tested the hypothesis that under conditions inducing severe hyperthermia, caffeine exacerbates hyperthermia-induced hyperventilation and reduces the cerebral blood flow index during exercise.
Using a randomized, single-blind, crossover design, 12 physically active healthy young men (23 ± 2 yr) consumed a moderate dose of caffeine (5 mg·kg-1) or placebo in the heat (37°C). Approximately 60 min after the ingestion, they cycled for ~45 min at a workload equal to ~55% of their predetermined peak oxygen uptake (moderate intensity) until their core temperature increased to 2.0°C above its preexercise baseline level.
In both trials, ventilation increased and the cerebral blood flow index assessed by middle cerebral artery mean blood velocity decreased as core temperature rose during exercise (P < 0.05), indicating that hyperthermia-induced hyperventilation and lowering of the cerebral blood flow occurred. When core temperature was elevated by 1.5°C or more (P < 0.05), ventilation was higher and the cerebral blood flow was lower throughout the caffeine trial than the placebo trial (P < 0.05).
A moderate dose of caffeine exacerbates hyperthermia-induced hyperventilation and reductions in the cerebral blood flow index during exercise in the heat with severe hyperthermia.
咖啡因是一种运动表现增强剂，广泛应用于在热应激条件下进行训练或比赛的个体。人们认为在热环境下运动时摄入咖啡因是安全的，因为它不会对体温反应、心率或体液状态产生很大影响。然而，目前尚不清楚咖啡因是否会影响热诱导性过度通气或脑血流指数的降低。我们检验了以下假设，即在诱导严重发热的条件下，咖啡因会加剧发热引起的过度通气，并降低运动时的脑血流指数。
采用随机、单盲、交叉设计，12 名身体活跃的健康年轻男性（23±2 岁）在热环境（37°C）下摄入中等剂量的咖啡因（5mg·kg-1）或安慰剂。摄入后约 60 分钟，他们以约 55%的预定峰值摄氧量（中等强度）进行约 45 分钟的自行车运动，直到核心温度升高至比运动前基线水平高 2.0°C。
在两项试验中，随着运动时核心温度的升高，通气量增加，通过大脑中动脉平均血流速度评估的脑血流指数降低（P<0.05），表明发生了发热诱导性过度通气和脑血流降低。当核心温度升高 1.5°C或更高（P<0.05）时，与安慰剂试验相比，咖啡因试验中的通气量更高，脑血流更低（P<0.05）。
中等剂量的咖啡因会加剧在严重发热的热环境中运动时发热引起的过度通气和脑血流指数的降低。
